The present invention relates to therapeutic methods for protecting the heart from ischemic injury. More specifically, the methods of the invention involve administering agonists and antagonists or binary conjugates thereof which selectively activate or inhibit adenosine receptors simultaneously thereby enhancing the protective effects of preconditioning and rendering the myocardium more resistant to ischemia.
Several publications are referenced in this application by numerals in parenthesis in order to more fully describe the state of the art to which this invention pertains. Full citations for these references are found at the end of the specification. The disclosure of each of these publications is incorporated by reference herein.
Adenosine is released in large amounts during myocardial ischemia and can mediate potentially important protective functions in the cardiovascular system (1,4,5,7,9,14,17,18,19,25). Previous studies have shown that adenosine receptor agonists can precondition the heart when given before the onset of ischemia (4,5,9,14,17,18) and can cause reduction in infarct size or improvement in left ventricular function when given during reperfusion (1,19) or during both low-flow ischemia and reperfusion in isolated perfused heart (6,21,22). While activation of adenosine A1 and A3 receptors has been shown to mimic the cardioprotective effect of preconditioning (3,10,23,24), their roles in mediating the protective effect of adenosine administered during ischemia have not yet been fully elucidated. Further, the cardioprotective effect of exogenous adenosine infused during ischemia in the intact heart may be exerted at the level of coronary vasculature, circulating neutrophils, or cardiac myocytes.
Our previous studies have characterized a cardiac myocyte model of injury, which is induced by exposure of myocytes to prolonged hypoxia in glucose-free media (16,23). Use of this model has facilitated the identification of compounds that enhance the protective effects of preconditioning and also increase myocardial resistance to ischemia.
The present invention provides methods for preventing or reducing ischemic damage of the heart. In conducting research leading up to this invention, it was discovered that simultaneous activation of A3 and A1 receptors enhances the protective effects of preconditioning and increases myocardial resistance to ischemia. The concept underlying the present invention is the use of specific agonists which simultaneously activate these two adenosine receptors. Concomitant activation of the two receptors is believed to produce a synergistic effect enhancing the cardioprotective effects of preconditioning and increasing myocardial resistance to ischemia.
According to a preferred embodiment, the invention involves administration of specific A3/A1 agonists, such as N6-(2-trifluoromethyl)(carbamoyl)adenosine-5xe2x80x2uronamide or N6-(3-iodophenyl)(carbamoyl)adenosine-5xe2x80x2uronamide during ischemic attacks, or at risk for ischemic damage. The agonists of the invention may be delivered prior to a surgical procedure. They may also be administered to a patient to prevent or reduce the severity of ischemic damage during surgery. Additionally, the A3/A1 agonists may be administered following surgical procedures to reduce the risk of post-surgical ischemic complications. Finally, the A3/A1 agonists may be administered to patients with angina or to patients during a myocardial infarction. The angina may be chronic and stable, unstable, or post-myocardial infarction angina.
In yet another embodiment of the invention, a series of water-soluble MRS compounds are contemplated to be within the scope of the present invention. These compounds selectively activate the A3 receptor. Because the compounds of the invention do not cross the blood-brain barrier, the deleterious effects associated with A3 receptor activation in the brain are avoided. The MRS compounds will be used in conjunction with the A1 agonists of the invention to prevent or reduce ischemic damage to the heart.
Another preferred embodiment of the invention comprises novel binary conjugates which bind two adenosine receptors simultaneously. Exemplary binary conjugates of the invention contain moieties that act as agonists at both of the A1 and A3 adenosine receptors, such as MRS 1543. A second exemplary conjugate, MRS 1528, acts simultaneously as an agonist at the A3 receptor and as an antagonist at the A2a receptor. Methods are disclosed herein for the administration of these binary conjugates to protect the heart against ischemic damage.
Methods of simultaneous administration of the A3 and A1 agonists or the binary A3 agonist/A2a antagonist or the binary A3 agonist/A1 agonist of the invention include direct perfusion of the organ during surgery and intravenous administration. Additionally, the agonists and antagonists of the invention may be administered to patients in tablet form in an amount effective to prevent or reduce ischemic damage to the heart.
In yet another aspect of the invention, recombinant myocytes are provided which may be used to advantage in assessing the activity of agents that may possess cardioprotective activity. Cardiac myocytes may be transfected with any of the adenosine receptor encoding cDNAs and used to screen for novel therapeutic agents.
In a further aspect of the invention partial agonists at the adenosine A1 and A3 receptors are also effective at mediating cardioprotection. Such partial agonists and methods of use thereof are also within the scope of the present invention.
In yet another aspect of the invention, methods of administering the agents of the invention for the prevention of myocardial apoptotic cell death are also provided.